Electrostatic coating is usually obtained from a sprayhead that generates droplets in the range of about 10 micrometers (.mu.m) to 500 .mu.m. Most often the goal is to create a uniform coating that is several tens to several hundreds of micrometers thickness. For these coatings, droplets land on top of other droplets on a substrate and coalesce to form a continuous coating.
In conventional electrostatic spraying the droplets are generated from a liquid which, under electrical stress, dispenses the droplets from points of stress. Many of these electrostatic spraying processes generate droplets by first creating a liquid filament from each point of maximum electrical stress. When an electrostatic spraying process operates in this filament regime the operation can be further classified based on the flow rate in an individual filament of the liquid. At very low flow rates an electrospray mode occurs. In the electrospray mode the filament emanates from a liquid cone and the cone and filament can be fixed in space if the liquid cone is attached to a fixed structure such as the tip of a needle or other object. In the electrospray mode Rayleigh capillary or filament breakup is believed to occur, causing the tip of the filament to break up into a fine mist of droplets. As the flow rate to a filament is increased a flow rate is reached where the cone tip begins to take on a transparent look although the base of the liquid cone remains more opaque. Usually this can only be seen by use of an optical magnifier such as by viewing the liquid cone and filament through a cathetometer. This flow rate marks the beginning of the flow rate range where the filament operates in what is known as the harmonic spraying mode. If the flow rate of the filament is increased in the harmonic spraying mode the filament appears to become larger in diameter. Eventually, as the flow rate is increased further the transparency of the cone tip starts to disappear and with further increase in flow rate the filament becomes quite long and rather large in diameter. This flow rate where the transparency of the cone tip starts to disappear marks the beginning of the high flow rate mode. In summary, when an electrostatic spraying process is operated in the filament regime it can be classified according to its flow rate as operating either below, in, or above the harmonic spraying mode depending on the flow rate that occurs in a single filament. For a given liquid the actual flow rate range for the harmonic spraying mode is dependent on the liquid's properties, and especially the electrical conductivity. A large number of liquids useful in coating applications have their electrical conductivity in the range between 0.1 and 1000 microsiemens per meter (10.sup.-7 and 10.sup.-3 S/m). For liquids in this conductivity range the most conductive liquids start harmonic spraying when the filament flow rate reaches around 0.1 to 1 milliliter per hour (ml/hr) whereas for the least conductive the harmonic spray mode does not first occur until the filament flow rate reaches around 10 to 100 ml/hr.
Sample and Bollini (Journal of Colloid and Interface Science Vol. 41, 1972, pp 185-193) describe the harmonic spraying cycle and point out that at the start of the cycle the electrically stressed liquid first becomes elongated. Then, the liquid forms a cone shape which then develops a filament of liquid from the tip of the cone. The liquid filament elongates or stretches, and finally the liquid filament snaps off of the cone shaped base. This last step produces a free liquid filament which, due to the surface tension force, becomes a droplet, and a cone shaped liquid which, due to the surface tension force, attempts to relax back to its original state. However, during the cone's relaxation the imposed electrical stress starts another cycle of harmonic spraying. When viewed with optical magnification, the cone appears as an opaque liquid hemisphere inside a partially transparent cone with a filament nearly fixed in place. The cone's transparent property is due to the fact that during a portion of the time there is actually nothing present in that space since the liquid is relaxing back after the filament of liquid snapped off. As suggested by Sample and Bollini, if care is taken to control the initial amount of liquid from which electrical harmonic spraying occurs then the droplets generated from the filaments can be fairly close in size. When the flow rate is increased above the range where harmonic spraying occurs the length of the filament increases and Rayleigh capillary (or filament) instability begins to compete as a mechanism for breaking the filament into droplets. At these higher flow rates long filaments and large droplets are produced. In conventional electrostatic atomization the flow rate is usually operated in either the harmonic spray mode or in the higher flow rate mode. However, if the flow rate becomes too high only streaks of liquid are produced. In conventional electrostatic spraying no special care is taken to insure the droplets are the same diameter. However, because the electrical stress is reasonably constant, the droplets produced usually have a tighter size distribution than found in most non-electrostatic spray devices.
If the flow rate in a conventional electrostatic sprayhead is reduced below the harmonic spraying mode while the speed of the object being coated remains the same, the coating thickness is reduced, and eventually, at a low enough flow rate the coating loses its uniformity. Close examination shows that while some filaments are being developed in the electrical harmonic spraying or pulsing mode, other filaments start to develop from liquid cones which temporarily become fixed in space. Although such a liquid cone and its filament becomes temporarily fixed, droplets are still generated from the filament tip. The liquid filament has fluid flow within it and for a certain flow rate range the filament is unstable. Subsequently, the filament tip breaks-up into droplets due to Rayleigh capillary or filament instability. At this low flow rate both the filament that is produced and its droplets have a diameter quite small compared to the filaments and droplets produced at the high flow rate mode. For liquids useful in industrial coating applications, this low flow rate range typically occurs below about 0.1 to 100 milliliters per hour per filament depending on the fluid properties, and this low flow rate mode is called the electrospray mode. The electrospray mode produces droplets having uniform diameter, i.e., a narrow size distribution, in the 1 to 50 .mu.m size range depending on the properties of the liquid, the potential applied to the liquid and the flow rate. Whereas the high flow rate mode produces droplets typically above 50 .mu.m in diameter, the electrospray mode produces a fine mist. In general, electrostatic atomization or electrostatic spraying from filaments can be defined to include the electrospray mode, the harmonic spraying mode, and the high flow rate mode. The electrospray mode is only practical when very low flow rates are desired, as for example to produce thin coatings.
U.S. Pat. No. 2,695,002 (Miller) describes the use of an electrostatic blade and teaches atomization of a liquid at the blade edge. Later, the same inventor disclosed a picture of a device purporting to generate evenly spaced filaments of liquid emanating from a blade tip (Electrostatics and its Applications (1973) pp 255-258). These filaments were designed to produce a mist of fine droplets and the blade was disclosed as a way to generate a series of filaments which operate in the electrospray mode and in the harmonic mode. Regardless of the disclosures, one skilled in the art quickly learns that these filaments tend to dance and drift in time. Indeed it is very difficult to keep the filaments both spatially and temporally fixed. Furthermore, two adjacent filaments can drift apart causing a decrease in the atomized mist at that location. Likewise, two adjacent filaments can drift together causing a temporary increase in the atomized mist at that location. When the mist is applied to a substrate, this can cause decrease or increase in the coating thickness respectively.
The present invention relates to an electrostatic spraying process which is unlike many conventional electrostatic processes which have been used for a number of years to make reasonably thick coatings, e.g., several tens to several hundreds of micrometers. The present invention can be used to make uniform coatings, either discontinuous or continuous as desired, between about one tenth and several tens of micrometers. The present invention can operate in a stable state in the electrospray range. The electrospray range refers to a restricted flow rate range where a single liquid filament can be generated and controlled to produce a uniform spray mist. The total flow rate is then the sum of the flow rates of the individual filaments produced. The electrospray range is useful for generating a mist that can be used to produce a thin film coating. However, for the coatings to be uniform the mist must be uniform, which requires the filaments to be both spatially and temporally fixed. Much of the recent patent art is dedicated to the development of sprayheads which attempt to meet this criteria. The recent patent art has attempted to fix the number of filaments by causing the spray to occur from a fixed number of points such as needles or teeth. For example, U.S. Pat. No. 4,748,043 (Seaver et al.) discloses the use of a low density series of needles to create the series of filaments needed to coat very thin coatings in an electrospray coating process. U.S. Pat. No. 4,846,407 (Coffee et al.) discloses placement along a blade a series of sharp pointed protrusions which resemble teeth to overcome the filament movement problem. U.S. Pat. No. 4,788,016 (Colclough et al.) discloses a non-conductive blade with teeth and U.S. Pat. No. 4,749,125 (Escallon et al.) discloses shims which have teeth-like structures from blunt to sharp. While these devices do fix the number of filaments, they severely restrict the range of coating that can be accomplished without mechanically changing the coating head. Furthermore, devices which are made with fixed points can, at a certain voltage, give rise to a loss of uniform mist when multiple filaments start to occur at one point and a single filament occurs at an adjacent point.